1. Field of the Invention
The present invention relates to the area of image signal processing, and more particularly to techniques for detecting a pulldown field sequence.
2. Description of Related Art
To satisfy the broadcast and display requirements of television programs, a progressive scan frame signal should be converted into an interlaced scan field signal. There are two traditional methods to solve this problem. One of the methods is that either all even line data or all odd lines data in each frame image is reserved as single-field data with the rest of the lines data discarded. As a result, a sequence of field data is generated and considered as a normal field sequence. The other one of the methods needs to employ a pulldown conversion technique which depends on a ratio of inputting frame ratio N to outputting field ratio M. Referring to FIG. 1, the most popular one is a 2:2 pulldown conversion, which means each frame image is divided into one odd field data and one even field data with the lines of the odd field interleaved with the lines of the even field. The alternating odd and even fields produced by the 2:2 pulldown conversion is named as a 2:2 pulldown field sequence.
The 2:2 pulldown conversion is adapted on condition that the field ratio M is twice of the frame ratio N or nearly twice. For example, when a film program running at 24 frame/second needs to be played on a television device in PAL standard which operates at 50 field/second, the following operation should be performed: converting the film program with 24 frame/second into the 2:2 pulldown field sequence with 48 field/second; broadening the 2:2 pulldown field sequence with 48 field/second into the field sequence with 50 frame/second; playing the field sequence with 50 frame/second on the TV device in PAL standard.
On the other end, in order to make up the lost odd or even lines data in the interlaced scan field signal and satisfy the resolution requirement, the interlaced scan field signal should be de-interlaced into the progressive scan frame signal before being played back on a progressive scan display device. There are a lot of conventional ways to de-interlace the interlaced scan field signal, such as an intra-field processing method, an inter-field processing method and a motion-adaptive method and etc. It is also well known that each of the de-interlaced methods may perform differently with respect to a different type of interlaced scan field signals. For example, each field of a normal field sequence records image information at a unique time with one half of the information lost, hence there is a need to recover the lost information when de-interlacing the normal field sequence. While the pulldown field sequence records integral image information, the de-interlaced method therefor may be quite different from that for the normal field sequence.
The interlaced scan signal in common use is a mixed video stream which not only comprises a pulldown field sequence but also a normal field sequence, hence a different de-interlaced method requires to be selected for the pulldown field sequence and the normal field sequence.
Therefore, there is a need for distinguishing the pulldown field sequence from the normal field sequence.